Efficient HIBE with Constant Size Ciphertext and Authorized Delegation

In this paper, a new technique - Identifier Discrimination is proposed for composing private keys in hierarchical identity based systems, With the technique, we construct a selective identity secure HIBE system under Decisional Bilinear Diffie-Hellman (DBDH) assumption in standard security model, where the ciphertext and the private key consist of constant number of group elements, and decryption requires only three bilinear map computations, regardless of the identity hierarchy depth. Moreover, different from previous HIBE constructions, key escrow problem inherent in identity based cryptosystems is resolved in our HIBE construction. An entity in hierarchy can be authorized by the root PKG to be capable of deriving private keys for its descendants. That we call Authorized Delegation.